1. Field of the Invention
The present invention relates to an apparatus for fractionating a predetermined component from a continuously supplied liquid. More particularly, the present invention relates to a liquid fractionation apparatus which is preferably used, for example, for continuously fractionating a plasma component from blood continuously collected from a living body.
2. Description of the Related Art
In this specification, the term "liquid" denotes a cell suspension.
A known means for fractionating a liquid is a separating method using a separating membrane, and a specific apparatus for separating a plasma component from blood is disclosed in Japanese Unexamined Patent Publication No. 58-121956 (EP-85016-A or Japanese Unexamined Patent Publication No. 59-155758 (EP-112152-A.sub.2).
The apparatus disclosed in the former reference (hereinafter referred to as "first conventional technique") utilizes an ordinary static membrane separation method and the apparatus disclosed in the latter reference (hereinafter referred to as "second conventional technique") uses this membrane separation method at the field where positive action is effected by a high shear rate and a centrifugal force. In order to produce this field at which such a positive action of the shear rate and centrifugal force is effected, there are used a secured cylindrical housing and a rotary spinner having a separating membrane bonded to the periphery thereof and rotated within the cylindrical housing at a certain distance from the housing wall.
An academic investigation of the shear rate to be applied in the separation of plasma from blood is reported in "Plasma-peresis", pages 135-143, published in 1983 by Raven Press, New York, and an experimental apparatus used during this investigation is illustrated in FIG. 3. This experimental apparatus (hereinafter referred to as "third conventional technique") comprises a blood cup for containing blood therein, a plane separating membrane secured to the lower portion of the blood cup, a space for receiving and discharging the plasma component provided below the separating membrane, and a conical body convex to and disposed rotatably above the separating membrane, spacedly therefrom within the blood cup. In this experimental apparatus, an amount of blood necessary for one experiment is contained in the blood cup.
In the first conventional technique using the static membrane separating method, a hollow fiber bundle comprising many hollow fibers is used as the separating membrane, and a separating surface area of 1000 to 3000 cm.sup.2 is necessary. Accordingly, the first conventional technique is disadvantageous in that the size of the apparatus is inevitably increased, the structure of the apparatus is complicated, and since large quantities of hollow yarns are used, the apparatus becomes expensive. Moreover, the amount of liquid resident in the apparatus is large in proportion to the large separating surface area, and especially when blood is the object of the treatment, the quantity of the blood withdrawn from the body is increased and there arises another serious problem in that the properties of blood are changed by contact between a large quantity of blood and the separating material, which is a material different from the living body. It is considered in principle that the blood flow rate through the hollow fibers is increased to increase the shear rate. In this case, however, the flow resistance is abruptly increased upon circulation through the hollow fibers, and the pressure supplied to the apparatus must be proportionally increased, with the result that a serious accident such as destruction of blood cellular components or flow-out of blood by breakage of the hollow fibers is readily caused.
In the second conventional technique using the membrane separation method under the action of the high shear rate and centrifugal force, in order to produce the field at which both the shear rate and the centrifugal force act, it is necessary to increase the diameter of the rotary spinner, and a problem of an increase in the size of the apparatus arises, and thus a structural problem arises in that the separated plasma must be taken out through the rotation shaft of the rotary spinner. More important is that, since centrifugal force is utilized, Taylor vortexes are readily generated in the liquid-treating zone of the apparatus, and especially where the liquid to be treated is blood, the turbulence of the blood flow due to Taylor vortexes induces the destruction of platelets. Moreover, since excessive pressure must be applied to the separating membrane against the centrifugal force, an excessive supply pressure is proportionally provided and an accident is likely involving a flow-out of blood by breakage of the introduction tube.
A batchwise apparatus is used in the third conventional technique, but in this technique blood is subjected to a shear rate in the blood cup for a long time and, therefore, a problem of destruction of platelets arises. Furthermore, this is an open system apparatus in which blood to be treated is contained in a blood cup which is exposed to the open air. Accordingly, in a continuous treatment blood, a problem of the air content of blood arises. Moreover, this conventional technique is defective in that a pump must be provided for each system including the supply system, the discharge system, and the filtrate system, and thus it is impossible to set the filtration pressure in an optional manner.